Track displacements are known that can be used as indicators of the conditions of a track on which railroad vehicles travel. Five track displacements are known: gauge displacement, cross level displacement, longitudinal level displacement, alignment displacement, and track twist. When such track displacements increase, railroad vehicles travelling thereon shake more such that the passengers feel uncomfortable. In addition, when track displacements increase, the travel safety of railroad vehicles may be impaired, potentially causing a derailment. Accordingly, the track displacements should be measured on a regular basis and the track should be repaired at an appropriate time.
A track twist indicates a twist relative to the plane of a track, and means the difference between the cross level displacements of two points separated by a predetermined distance along the length direction of the track. Cross level displacement means the difference between the heights of the left and right rails making up the track.
Traditionally, track displacements are measured by a special track inspection car (for example, see JP 2001-130408 A). Track inspection cars are not for commercial operation, and are only available in small numbers. As such, track displacements cannot be measured frequently, and must be measured in a non-operation period, such as in the middle of the night. Thus, needs exist for the ability to measure track displacements frequently using a vehicle for commercial operation instead of a track inspection car.
Meanwhile, it has been proposed to use, in a vehicle for commercial operation, a bogie capable of measuring wheel load and lateral force (PQ monitoring bogie) to frequently measure derailment coefficient, which is an indicator of the travel safety of a railroad vehicle (for example, see JP 2006-88967 A, and “Method of Measuring Wheel/Rail Contact Force Without Using PQ Wheel Axel”, Collected Papers from the Japan Society of Mechanical Engineers (Compilation C), Vol. 77, No. 774 (February 2011), pp. 147-155).
Wheel load is a vertical force acting between a wheel of a railroad vehicle and a rail of a track. Lateral force is a horizontal force (i.e. force in a direction along the axles) acting between the wheel and rail. Derailment coefficient is an indicator represented by Q/P where P is the wheel load and Q is the lateral force.